U.S. Pat. No. 4,341,953, issued Jul. 27, 1982 to Sakai et al. discloses a focus detection system having a plurality of linear image sensing arrays and a beam splitter for forming three images on the arrays, one of which is at nominal best focus, one slightly in front of best focus, and one slightly behind best focus. Signal processing electronics receive the signal from the image sensors and determine a position of best focus as a function of the variance of the intensity of the sensed images. This approach has the draw back that the one dimensional image sensors provide a limited sampling of the image. A larger sampling of the image is desirable since the signal to noise ratio of the focus signal is proportional to the square root of the number of pixels in the sample.
It is also known to use an area array image sensor in a camera and move the lens through its focus range while capturing a number of images. This approach has the advantage over a linear sensor in that the sampled image is inherently more representative of the actual image to be recorded and the focus determination is more accurate due to the larger sampling of the image, however it is not applicable if the image is moving, and the focus sensor has moving parts, which is also undesirable from a reliability standpoint. It would therefore be desirable to employ an area array image sensor in a focus sensor of the type shown by Sakai et al. However, the prism arrangement in Sakai et. does not allow for simultaneous two-dimensional image fields and small focus differentials. If the image sensors in Sakai et al. are spaced apart along the optical axis, as they would be if they were area array sensors, the differences in focus position would be prohibitively large. As shown in FIG. 6, in a focus sensor of the type disclosed by Sakai et al. generally designated 10, having beam splitting prisms 12, 12' and 12" and adapted to employ area array image sensors 14, 14', and 14", if the focus position 16' on the second image sensor 14' is chosen to be near optimum, the focus position 16 on the first image sensor will be too far below the sensor, and the focus position 16" on the third image sensor will be too far above the image sensor 14". In general, for the optical arrangement disclosed by Sakai et al the focus differentials will be roughly equal to two thirds the image sensor spacing if glass prisms are used or one times the image sensor spacing if thin plate beam splitters are used. The desire for large area arrays and small focus differentials are thus mutually exclusive.